Introduction: The Distal metatarsal articular angle (DMAA) is a radiographic measure of orientation of the first metatarsal articular surface, it is frequently used in the management of hallux valgus. There is a great deal of conflict regarding accuracy, reproducibility and validity of the DMAA within the literature. This study aims to test the validity of the measurement of the DMAA from standard radiographs, to explore the trigonometric relationship of first metatarsal rotation and the DMAA and to assess inter-observer reliability.

Materials/Methods: 34 seperate dry cadaveric first metatarsal bones were mounted onto a customized light-box/protractor allowing controlled incremental changes in rotation and inclination. A series of 39 digital photographs were taken of each metatarsal in 5 degree increments of rotation between 30 degrees supination and 30 degrees pronation and 10, 20 and 30 degrees of inclination. Three reviewers performed blinded DMAA measurements from each image; the data was collated for statistical analysis.

Results: The data was analysed using a mixed effects linear model comparing the DMAA with rotation of the first metatarsal. A strong statistically significant trend of increasing score with increasing pronation is observed, the relationship of which is approximately linear. There is a strong effect of inclination, but the strength of this varies with rotation this is amplified at higher inclinations. Inter-observor error was noted in line with other studies, the linear relationship is maintained.

Discussion: This study has shown that the distal metatarsal articular angle varies significantly, in an almost linear pattern, with axial rotation of the first metatarsal. Inclination of the first metatarsal is also shown to affect the magnitude of the angle.

This study does not refute the distal metatarsal articular angle as an entity, but does confirm the inaccuracy of extrapolating the DMAA from plain AP radiographs.